The state of the art in optical communication networks, particularly that related to photonics based components for use in such networks, has advanced rapidly in recent years. Present applications require, and future application will demand, that these communication systems have the capability to reliably transfer large amounts of data at high rates. Moreover, because these networks need to be provided in a cost efficient manner, especially for “last mile” applications, a great deal of effort has been directed toward reducing the cost of such photonic components while improving their performance.
Typical optical communications systems use fiber optic cables as the backbone of the communication system because fiber optics can transmit data at rates that far exceed the capabilities of wire based communication networks. A typical fiber optic based communication network uses a transceiver based system that includes various types of optoelectronic components. Generally, a transceiver includes a light source, means to convert an electrical signal to an optical output signal, and means to convert an incoming optical signal back to an electrical signal. A laser is used to provide the source of light and a modulator is used to turn the light source into an information bearing signal by controllably turning the light on and off. That is, the modulator converts the light from the laser into a data stream of ones and zeroes that is transmitted by a fiber optic cable. The incoming optical signal can be converted back to an electrical signal by using components such as amplifiers and photodetectors to process the signal.
Commercially used optical modulators are either lithium niobate based devices or compound semiconductor based devices such as the III-V based devices that use gallium arsenide or indium phosphide material systems. Additionally, silicon based devices have been developed. However, silicon based optical modulator technology has not been able to provide a device that can perform like the commercially available products and many problems need to be solved before such silicon based devices can compete with the commercially available lithium niobate and compound semiconductor devices.
Lithium niobate devices rely on an electrooptic effect to provide a modulating function. That is, an electric field is used to change the refractive index of the material through which the light is traveling. These devices are usually provided as a Mach-Zehnder interferometer. In this type of modulator, an incoming light source is divided and directed through two separate waveguides. An electric field is applied to one of the waveguides, which causes the light passing through it to be out of phase with respect to the light in the other waveguide. When the light emerges from both waveguides and recombines, it interferes destructively, effectively turning the light off.
In contrast, compound semiconductor based devices rely on an electroabsorption effect. In this type of modulator an applied electric field is also used, but not to vary the refractive index of the material through which the light is propagating. In a compound semiconductor material, an electric field can be used to shift the absorption edge of the material so that the material becomes opaque to a particular wavelength of light. Therefore, by turning the electric field on and off, the light can be turned on and off.
One problem with lithium niobate based modulators is that as the data transfer rate increases for these devices, so must the size of the device itself. This requires more material, which can increase cost. These modulator devices are often integrated into packages with other components where the demand for smaller package sizes is continually increasing. Therefore, modulator size is a concern. Another problem with lithium niobate based devices is that the drive voltage can be somewhat high as compared to compound semiconductor devices. Accordingly, because a large voltage change between the on and off state is more difficult to produce than a lower voltage swing, the drive electronics required to provide such large voltage changes are typically relatively expensive and can introduce more cost to the systems.
Compound semiconductor modulator devices can be made extremely small and are not limited by the size restrictions of lithium niobate based devices. Moreover, these devices can handle high data transfer rates at relatively low drive voltages. However, current compound semiconductor based modulators, such as those fabricated from the indium phosphide material system, have certain limitations. In particular, these devices can suffer from problems related to coupling losses and internal absorption losses, which are generally not present in lithium niobate based devices.
As an additional concern, the processing and manufacture of compound semiconductor based devices is expensive when compared to silicon based devices, for example. One reason for this is that many of the base materials used for compound semiconductor processing are expensive and difficult to handle. For example, indium phosphide wafers are presently limited in size and the largest wafers are expensive. This makes low cost high volume manufacturing difficult as compared to that which can potentially be obtained in the manufacture of silicon based devices.
Regarding silicon based technology, a silicon based modulator can be designed to function in a manner that is similar to the way a lithium niobate based device functions in that it changes the phase of the light passing through a waveguide. This phase change can be used in a Mach-Zehnder type device to form a modulator. More particularly, a silicon based device generally operates on the principle that a region of high charge concentration can be used to shift the phase of light in the waveguide. Importantly, the magnitude of the phase shift is proportional to the charge concentration and the length of the charged region in a direction in which the light travels. Thus, the ability to create a region of sufficient charge density to interact with the light is essential to be able to induce a phase change, especially one that can shift the phase by an amount suitable for use in a Mach-Zehnder type device.
In order to provide a charged region that can be used for phase shifting, these devices are known to use injection of electrons or depletion of holes in a diode or triode type device. In operation, a concentration of charge carriers can be provided in an active portion of a guiding region of a waveguide in these devices. One parameter that is important in a silicon based optical modulator is the speed in which a charged region can be created and subsequently dissipated. More particularly, the speed at which charge carriers can be generated as well as the speed at which charge carries can be removed (by recombination, for example) affects the speed at which modulation can be performed. These generation and recombination processes are directly related to the mobility of the charge carriers in the particular material. Because these devices use both single crystal silicon and non-single crystal silicon and because the mobility of charge carriers in non-single crystal silicon is significantly lower than the mobility of charge carriers in single crystal silicon, the low mobility non-single crystal material unfortunately limits the rate at which the device can modulate light.